In Omny Studio you'll find a suite of analytics and metrics. To go beyond download numbers, Omny also provides an estimate for your number of unique listeners, for the selected time period in clip, program, network or organization-level analytics.
You can find this figure from the top of your analytics pages in Omny, whether that's at the Clip/Program/Network/Organization level.
Just select the date range you want to see the Estimated uniques for.
Estimated unique listeners are "an estimated number of unique users who downloaded clips during the selected time period."
"Listener is an IAB metric represented by a combination of IP address and user agent."
Your Estimated unique listeners are a great way to go beyond your downloads to determining your audience size. Another analytics tool available to you is the data available from certain podcast apps, specifically Apple and Spotify;